The problem of exhaust gas emissions basically manifests itself as an excess of oxides of nitrogen (NO.sub.x) at high and full power operation and an excess of carbon monoxide (CO) at low power operation. Techniques have been produced which reduce the NO.sub.x emissions, but these have tended to increase the CO emissions and vice-versa.
The problem originates in the primary zone of the combustor where rapid reaction rates are achieved by using near-stoichiometric mixtures of fuel and air, as a result of which, the temperatures generated are sufficiently high to promote the formation of NO.sub.x. The maximum temperature can be reduced by operating at an off-stoichiometric mixture strength, the condition of mixture strength being known as the equivalence ratio (.phi.) which is defined as the ratio of fuel to air fractions between the operational and the stoichiometric conditions.
The effect of operating the primary zone at off-stoichiometric condition is that the formation of NO.sub.x can be significantly reduced providing that .phi. is greater than about 1.2 or less than about 0.8. The fuel rich solution leads to a large combustion chamber, and for aero-engines and their industrial derivatives it is necessary to take the fuel lean solution which in itself has adverse effects when the engine is operating at part load conditions. There is a tendency for .phi. and the compressor delivery air temperature to drop, resulting in the emission of large quantities of CO and the likelihood of combustion instability.
One solution to this problem is to use staged combustion in which low power requirements are met entirely by a first stage combustor running at near stoichiometric conditions, while at greater loads, the first stage is used as a torch to ensure stability in a larger, second stage combustor, operating at a lower equivalence ratio. This type of system can be difficult to manufacture, requires a large number of fuelling points and fuel injection systems using pre-mixing, and pre-vaporizing techniques may be required to make combustion adequately homogeneous.
Another solution is to vary the primary zone equivalence ratio by controlling the amount of air flowing into it, and a method and apparatus utilizing a fluidic control of air distribution is disclosed in the aforementioned United States patent application Ser. No. 827,109 (now abandoned) and its continuation-in-part application Ser. No. 62,418.
In those applications there was disclosed a combustion chamber having first and second air inlet means and a variable rate diffuser upstream of both said inlet means, the first air inlet means comprising a first annular duct defined by part of the wall of the combustion chamber and an intermediate casing, the combustion chamber having air inlets for the flow of primary and dilution air from the first annular duct, the second air inlet means comprising a second annular duct surrounding the first annular duct, the second annular duct including a part of the wall of the combustion chamber having air inlets for the flow of bypass air, the variable rate diffuser comprising vortex generating means and a variable rate air bleed, the variable rate diffuser receiving a supply of air and delivering the air to the first and second inlet means in a ratio dependent on the rate of diffusion therein.
The variable rate diffuser typically comprises a primary duct receiving a supply of air located within a secondary duct, a fence in the secondary duct downstream of the outlet of the primary duct and a bleed duct having a variable controllable bleed in the secondary duct upstream of the fence.